Diaphragm spring clutches have been known for a long time and, owing to their robust and space-saving structure, have become well established particularly in motor vehicles as the preferred design for passively engaging, dry-operating clutches. In a diaphragm spring clutch, the contact pressure spring is made as a diaphragm spring, by means of which a pressure plate is pressed axially against a counterpressure plate which is connected in a rotationally fixed manner to the driveshaft of the drive motor and is usually made as a flywheel, whereby at least one driving disk provided with friction linings and connected rotationally fixed to the input shaft of the change-speed transmission, is gripped so that torque from the drive motor can be transmitted to the manual transmission by the action of the friction force. The diaphragm spring is usually designed such that over the entire life of the clutch it ensures reliable torque transmission.
To engage and disengage a diaphragm spring clutch, the diaphragm spring has inner spring blades on which a clutch control element, made as a central release device or a release lever that can be pivoted by an externally arranged clutch control element, can exert an axial release force via a release bearing. As regards the structure of the diaphragm spring clutch, in particular the support of the diaphragm spring, a distinction can be made between an extended clutch, in which the releasing force acts in the direction of the transmission, and a compressed clutch, in which the releasing force acts toward the drive motor.
In the diagram of FIG. 4 the upper curve shows the contact pressure force FK—Anpr and the lower curve shows the releasing force FK—Ausr of a diaphragm spring, in each case plotted against the release travel path xK for the engaged condition of the clutch. On the upper curve the point PB indicates the operating point of the clutch for fresh friction linings of the associated driving disk and the point PB′ the operating point for worn friction linings. Consequently, in the case of a diaphragm spring clutch with automatic zero-point compensation the working range, i.e. the range between the “closed” path point xK0 for the fully engaged condition and the “open” path point xK1 for the fully disengaged condition, becomes displaced during the course of friction lining wear from right to left, i.e. in the representation of FIG. 4 from the range xK0 to xK1 to the range xK0 ′ to xK1′. The corresponding variations of the releasing force against release travel path during a clutch release process are shown in simplified form in FIG. 4, respectively as broken lines FKA and FKA′ under the curve for the releasing force FK—Ausr.
A detailed representation of a typical variation of the releasing force FKA against release travel path xK during a disengagement process of a diaphragm spring clutch is reproduced qualitatively in FIG. 5. According to this the variation of the releasing force, FKA is divided into an initial, rising range A in which the releasing force FKA increases almost linearly as a function of release travel path xK, followed by a transition range B with a progressively slowing increase of the releasing force FKA to a value which, in a subsequent saturation range C, remains essentially constant and in the range of the maximum releasing force FKA—max that can be reached there during normal service operation. In the next range D the releasing force FKA at first decreases with increasing release travel path xK, and then increases again.
In view of the non-linear variation of the releasing force FKA (xK) and of production-tolerance-related deviations and wear-related changes of the variation, there is a need for a corresponding adaptation of control parameters of the associated clutch control element in order to ensure reproducibility of automatically controlled disengagement and engagement processes. Among other things this is also made clear by the fact that in DE 10 2005 039 922 A1 a control element of a diaphragm spring clutch is proposed, such that the characteristic regulation curve of an associated regulator is designed so that the non-linear variation of the releasing force FKA(xK) is compensated by appropriate actuation of the clutch control element. However no possible method for adapting the characteristic regulation curve, i.e. for adapting the characteristic regulation curve to the respective releasing force variation FKA(xK) of the diaphragm spring clutch at the time, is described in DE 10 2005 039 922 A1.
Known adaptation methods for automated friction clutches have hitherto been limited to the determination of inflection points on the respective torque characteristic representing the torque that can be transmitted by the friction clutch as a function of the regulating path. Thus for example, various methods are known for determining or adapting the “closed” point or engaged point on the torque characteristic, in which that value of the regulating path is determined, at which the friction clutch is completely engaged and a maximum torque predetermined by design can be transmitted.
In other known methods the active point of a friction clutch, also known as its touch or contact point, is determined or adapted, this being the point at which the frictional elements concerned just come into contact with or separate from one another so that the friction clutch can transit an infinitesimally small torque. Such methods are based on sensor-detectable reactions of operating parameters of associated or nearby components, such as the regulation path of the clutch control element, the speed of the transmission input shaft or the drive motor, or the fuel injection quantity of the drive motor.
In contrast, in DE 101 63 438 A1 a method is described, in which during an automated clutch actuation of a diaphragm spring clutch an operating parameter is detected by sensor means, and from the variation of this operating parameter an adaptation parameter is derived for correcting a control parameter of the associated clutch control element. According to this, it is provided that during a disengagement or engagement process of the friction clutch the releasing force FKA is determined by sensor means as a function of the release travel path xK, and from the variation of the releasing force KKA(xK), specifically with reference to a discontinuity in the gradient variation of the releasing force FKA(xK), the contact point of the diaphragm spring clutch is determined. The disadvantage of this procedure, however, is that besides a path sensor an additional force sensor is needed, and the discontinuity in question in the gradient variation of the releasing force FKA(xK) only occurs in significant form when the driving disk has a rigid lining with no spring. In any case, in this method the variation of the releasing force FKA(xK) is not evaluated for its own sake, but is only used for determining the contact point which, however, is only one inflection point of the torque characteristic.